1. Field of the Invention
This invention relates generally to parabolic dish structures and more particularly to a novel segmental parabolic dish and a method of constructing the dish from relatively thin flexible sheet material. The invention relates also to a novel polarizing parabolic dish antenna reflector having a polarizing grid directly on the parabolic surface of the reflector dish and to a method of constructing the reflector utilizing the segmental parabolic dish technique of the invention.
2. Prior Art
In the context of the present invention, a parabolic dish is essentially a relatively thin-walled shell-like structure having the shape of a paraboloid. The dish may be either symmetrical or non-symmetrical about its principle axis. Such a parabolic dish may be utilized for a variety of purposes, and, in its broader aspects, this invention is concerned with providing a segmental parabolic dish which may be used for any of these purposes. In its more limited aspects, however, the invention is concerned with parabolic dish-type antenna reflectors and will be described in connection with this particular application.
A parabolic dish antenna comprises, essentially, a parabolic reflector dish and an antenna feed at the focal point of the reflector. The prior art is replete with a vast assortment of such antennas and reflector dishes and techniques for their fabrication. In some cases, the reflector dish is collapsible for storage in minimum space and in other cases is a rigid structure. This invention is concerned with such antenna reflectors.
One method of forming such a parabolic reflector dish involves forming from sheet material, such as fiber glass cloth, a plurality of sections or gores which may be assembled to form a parabolic reflector dish. These dish sections, or gores, may have various gore shapes such as triangular and circular. This method of reflector dish fabrication is quite satisfactory for many parabolic antenna applications but is not suitable to the particular parabolic antenna application with which the present invention, in its more limited aspects, is concerned.
Thus, these more limited aspects of the invention are concerned with a so-called polarizing or polarized parabolic dish antenna for producing a radiation beam which is polarized in a given direction or plane. This type of antenna is useful on a communications satellite, for example, for the reason that two antennas, with different directions of polarization, may be utilized to beam transmission of the same carrier frequency to two contiguous regions of the earth without interference between the two transmissions, thus effectively doubling the communications capacity of the satellite.
One method of accomplishing such antenna polarization involves mounting of a polarizing grid, consisting of spaced parallel conductors, in front of the antenna reflector dish. This type of polarizing antenna has certain disadvantages which restrict its use. Perhaps one of the foremost disadvantages resides in the fact that outboard placement of the polarizing grid in front of the reflector dish introduces undesirable constraints into the relative positioning of two differently polarized antennas which may preclude placement of the two antennas in the most favorable relative positions. Moreover, this polarizing grid arrangement requires a grid support which increases the antenna weight and complexity and introduces an additional unreliability factor which must be considered.